Tissue ischemia and sometimes tissue death are the result.
In most people with decompression sickness, the symptoms are pain in the joints and muscles of the legs and arms, affecting 85 to 90 per cent of those persons who develop decompression sickness. The joint pain accounts for the term "bends" that is often applied to this condition.
In 5 to 10 per cent of people with decompression sickness, nervous system symptoms occur, ranging from dizziness in about 5 per cent to paralysis or collapse and unconsciousness in as many as 3 per cent. The paralysis may be temporary, but in some instances, damage is permanent.
Finally, about 2 per cent of people with decompression sickness develop "the chokes," caused by massive numbers of microbubbles plugging the capillaries of the lungs; this is characterized by serious shortness of breath, often followed by severe pulmonary edema and, occasionally, death.
Nitrogen Elimination from the Body; Decompression Tables. If a diver is brought to the surface slowly, enough of the dissolved nitrogen can usually be eliminated by expiration through the lungs to prevent decompression sickness. About two thirds of the total nitrogen is liberated in 1 hour and about 90 per cent in 6 hours.
Decompression tables have been prepared by the U.S. Navy that detail procedures for safe decompression. To give the student an idea of the decompression process, a diver who has been breathing air and has been on the sea bottom for 60 minutes at a depth of 190 feet is decompressed according to the following schedule: 10 minutes at 50 feet depth 17 minutes at 40 feet depth 19 minutes at 30 feet depth 50 minutes at 20 feet depth 84 minutes at 10 feet depth Thus, for a work period on the bottom of only 1 hour, the total time for decompression is about 3 hours.
Tank Decompression and Treatment of Decompression Sickness. Another procedure widely used for decompression of professional divers is to put the diver into a pressurized tank and then to lower the pressure gradually back to normal atmospheric pressure, using essentially the same time schedule as noted above.
Tank decompression is even more important for treating people in whom symptoms of decompression sickness develop minutes or even hours after they have returned to the surface. In this case, the diver is recompressed immediately to a deep level. Then decompression is carried out over a period several times as long as the usual decompression period.
"Saturation Diving" and Use of Helium-Oxygen Mixtures in Deep Dives. When divers must work at very deep levels— between 250 feet and nearly 1000 feet—they frequently live in a large compression tank for days or weeks at a time, remaining compressed at a pressure level near that at which they will be working. This keeps the tissues and fluids of the body saturated with the gases to which they will be exposed while diving. Then, when they return to the same tank after working, there are no significant changes in pressure, so that decompression bubbles do not occur.
In very deep dives, especially during saturation diving, helium is usually used in the gas mixture instead of nitrogen for three reasons: (1) it has only about one fifth the narcotic effect of nitrogen; (2) only about one half as much volume of helium dissolves in the body tissues as nitrogen, and the volume that does dissolve diffuses out of the tissues during decompression several times as rapidly as does nitrogen, thus reducing the problem of decompression sickness; and (3) the low density of helium (one seventh the density of nitrogen) keeps the airway resistance for breathing at a minimum, which is very important because highly compressed nitrogen is so dense that airway resistance can become extreme, sometimes making the work of breathing beyond endurance.
Finally, in very deep dives it is important to reduce the oxygen concentration in the gaseous mixture because otherwise oxygen toxicity would result. For instance, at a depth of 700 feet (22 atmospheres of pressure), a 1 per cent oxygen mixture will provide all the oxygen required by the diver, whereas a 21 per cent mixture of oxygen (the percentage in air) delivers a Po2 to the lungs of more than 4 atmospheres, a level very likely to cause seizures in as little as 30 minutes.
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